Field of the Invention
The present invention relates to novel glasses with a high silica content as well as a significant content of additives such as ruthenia and other Group VIII metal oxides, and to methods of conveniently producing such glasses in porous form, in which they can be used in applications requiring a large surface area, such as chemical sorption, ion exchange, or catalysis. Porous glasses, including compositions containing high levels of silica, have been known for some time, but types and amounts of metal oxide additives which can be introduced into such compositions, as well as practical methods of introducing such additives, have been very limited. Introduction of metal oxides into the surface or porous structure of glasses permits useful variations of their properties. The present invention relates to novel methods which permit the introduction of significant amounts of metal oxide additives, variable over a broad range, into porous glass compositions. The method comprises melting a phase separable glass composition, for instance an alkali borosilicate glass, with a pre-selected amount of metal oxide additive. Upon heat treatment to separate the glass into a soft, leachable phase and a hard, non-leachable phase, a substantial amount of the metal oxide follows the hard phase, and accordingly it remains in the porous glass structure upon subsequent leaching in an aqueous, usually acidic, solution. Subsequently, the porous glass can be subjected to further treatments, for instance treated with alkali metal ions, ammonium ions to convert it into a highly effective ion exchange material. According to the present invention such treatments can also follow other methods of introducing the metal oxide additive, for instance introducing it from the solution used to leach the soft phase, especially in the case of species such as alumina which tend upon co-melting and phase separation to segregate to a large extent into the soft, leachable phase. One reported application of porous glasses is their use as ion exchangers, for instance as cation exchangers, following surface treatments with ionic solutions, as described in U.S. Pat. No. 4,469,628 (Simmons et al.). However, conversion of porous glasses into ion exchange materials following the introduction of an oxide additive, for instance by incorporating the additive in the hard phase of a phase separable glass followed by leaching, has not been reported. U.S. Pat. No. 4,659,512 (Macedo et al.) describes the preparation of an ion exchange material which comprises treating a porous support having interconnected pores, said porous support having interconnected pores, said porous support being a silicate glass or silica gel, with an organic amine, for instance a neutral or basic water-soluble alkylene amine such as triethylenetetramine. Ion exchange materials prepared according to this process are used to remove metal species such as cobalt from liquids, in particular from aqueous solutions. Again, the use of this process using porous silicate glasses with metal oxide additives as support has not been reported.
This invention further relates to the ion exchange separation of rare earth elements, actinides or mixtures thereof.
According to Jaffe, U.S. Pat. No. 2,897,050 the term "rare earths" is used to designate the group of elements between lanthanum, atomic number 57, and lutecium, atomic number 71, inclusive, and to these elements should be added yttrium, atomic number 39, and scandium, atomic number 21, which are nearly identical with rare earths in properties and usually occur together with them in natural deposits.
In addition, according to Jaffe, U.S. Pat. No. 2,897,050, since the rare earths are intimately mixed together in the natural states and have very similar chemical and physical properties, differing each from the other very slightly, they cannot be easily separated. A number of processes have been suggested for separating the rare earth elements. These include: fractional crystallization or precipitation, solvent extraction, and ion exchange. However, according to the same patent, all these methods are tedious and difficult to control.
The actinide elements constitute another group of important elements which resemble each other in chemical properties. They are also similar, in particular when ions of the same oxidation states are considered, to the rare earths.
The most widely used technique of separating the rare earths is solvent extraction, described by Peppard et al., U.S. Pat. No. 2,955,913, and by Chiola et al., U.S. Pat. No. 3,598,913.
Ion exchange offers significant advantages over solvent extraction, including a much smaller plate height, applicability to multi-element isolation and higher ultimate purity (Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd edition, Volume 19, Wiley-Interscience, New York, 1982). Ion exchange is not widely used, however, because it relies on elution alone to separate the various rare earths, and therefore many theoretical plates are necessary, requiring extremely fine control over operating conditions to prevent band broadening, as well as slow flow rates and limited influent concentration. Another limiting feature of the elution technique is that the order of elution is always determined by the relative magnitude of the stability constants of the complexes of the various rare earths, and that in all cases heavier lanthanides have higher stability constants and therefore desorb more rapidly, as described by Spedding et al., U.S. Pat. No. 2,798,789 and by Choppin et al., U.S. Pat. No. 2,925,431. For instance, Nd.sup.3+ is eluted before La3.sup.+.
It is an object of the present invention to provide high silica porous glass compositions with significant amounts of metal oxide additives.
A further object of the present invention is to provide a convenient method of making high silica glasses with significant amounts of metal oxide additives in the porous form.
Yet a further object of the present invention is to provide ion exchange materials or sorbents which have high capacity, high selectivity, high chemical durability, or any combinations of these properties.
Another object of the present invention is to provide a convenient method of making ion exchange materials based on high silica porous glass.
Another object of the present invention is to provide catalysts or chromatographic materials which have high efficiency, high selectivity, high chemical durability, or any combination of these properties.
Another object of the present invention is to provide durable porous glass materials which can be used as catalyst supports, enzyme supports, supports for chromatographic stationary phases, or supports for chemical reagents.
Another object of the present invention is to provide a method for making a corrosion resistant porous glass which can be used as a support for a dye, an enzyme, or a dye-enzyme combination in a sensor device for measuring chemical or physical properties in aqueous media over significant time periods.
It is an object of this invention to provide a process for the separation of rare earth values, actinides or mixtures thereof by ion exchange in which a substantial extent of separation takes place without the introduction of a complexing agent.
A further object of the present invention is to provide a process of selectively removing more strongly complexable rare earth ions from a stream containing less strongly complexable rare earth ions.
Yet a further object of the present invention is to provide ion exchange materials a process for separation of rare earths, actinides or mixtures thereof which uses high capacity, high selectivity, or a combination of these properties.
Another object of the present invention is to provide a process of producing highly pure lanthanum with a very low content of heavier rare earths for optical applications.
Yet another object of the present invention is to provide a method of producing highly pure individual rare earths or groups of rare earths.
In accordance with the present invention, porous glasses with transition metal oxide additives are prepared by introducing such oxides, or salts which produce them upon heating, into the original batch composition, for instance a silica-boron oxide-alkali oxide mixture, which is used to prepare a phase separable glass. The mixture is melted, cooled down to form a glass, and heat treated to separate a nonleachable, silica-rich, "hard" phase from a more soluble "soft" phase which contains considerably less silica than the hard phase. The soft phase is removed by leaching in an aqueous, usually acidic, medium. The crux of this embodiment of the invention is the selection of an appropriate transition metal additive and of an appropriate amount of such additive in the initial mix so as to cause, at the end of the leaching step, the majority or a substantial fraction of the additive to remain in or with the resulting porous glass formed by the undissolved hard phase. The resulting porous glasses can be used without further treatment, for instance as chromatographic stationary phases, sorbents, or catalysts, or chemically treated for such uses as ion exchange or supporting reagents, catalysts, chromatographic agents, enzymes, or indicator dyes. According to EP Application no. 85116188.5 (Beaver) Group IVa, but not Group VIII, oxides can be retained in the hard phase.
The incorporation of transition metal oxides which have catalytic properties in the batch composition of phase-separable glasses, followed by heat treatment and selective leaching of a soft phase, can be used to produce catalysts with a high specific activity. When such oxides, for instance ruthenia, remain to a significant extent upon phase separation in the hard phase or in the interphase between the hard and the soft phases, respectively, subsequent leaching leaves a large fraction of the additive oxide on the surface of the resulting porous glass, where it can come in contact with reactant molecules and cause effective catalysis of chemical processes. The processes of the present invention can be used to produce porous glasses with metal oxide additives such as the oxides of ruthenium, rhodium, palladium, rhenium, osmium, iridium, and platinum, which can exhibit catalytic activity ven when present at low concentrations.
Certain uses of porous glasses require a high degree of corrosion resistance. These include uses as chromatographic supports in media having high pH, high temperature or both, as well as uses as supports for indicator dyes in miniature probes, for instance probes used for in vivo blood analysis, which are highly susceptible to degradation by dissolution due to their small dimensions. Doping with a metal oxide additive such as zirconia prior to phase separation and leaching can lead to significant improvement in the chemical durability of the corresponding porous glasses.
Unlike other processes of producing porous glass with a high transition metal oxide content, which require the silica levels in the glass to be lower than 40 mol percent and the phosphorus oxide content to be at least 20 mol percent, according to U.S. Pat. No. 2,943,059, the current process is applicable to glasses with a high silica content and does not require the presence of phosphorus oxide.
Another embodiment of the present invention consists of producing an ion exchange material by treating a phase-separated glass with an aqueous, usually acidic, solution of a metal ion to effect simultaneous leaching of the soft phase and impregnation with a metal additive.
Porous glasses prepared by this process or by co-melting as described above can be activated, usually at moderately high pH, with an aqueous solution which contains alkali metal ions, ammonium ions, Group Ib ions, organic amines or combinations of any of these species produce highly effective ion exchange materials. The introduction of a metal oxide additive can lead to increased ion exchange capacity or selectivity as well as to improved corrosion resistance. The corrosion resistance controls ion exchanger performance in applications involving prolonged exposure to aqueous media or contact with corrosive media involving, for instance, a high pH value, a high temperature, or both.
The present invention comprises also a process for separating rare earth ions, actinides ions, or mixtures thereof in solution by passing the solution through an ion exchange material to separate the rare earths, actinides, or mixtures thereof. The ion exchange material has a surface area of about 5-1500 m.sup.2 /g. The ion exchange material may include from 0 to about 35 mol percent, preferably from 1 to 30 mol percent, of a metal oxide or hydrous metal oxide. The metal oxide or hydrous metal oxide is selected from the group consisting of the transition metals of Groups IIIa, IVa, Va, VIa, VIIa, VIII, Ib, and IIb of the Periodic Table, aluminum, gallium, indium, thallium, tin, lead, bismuth, beryllium, the actinides and mixtures thereof, preferably titania zirconia, hafnia, thoria and mixtures thereof. The ion exchange material is impregnated with a liquid containing alkali metal cations, Group Ib metal cations, ammonium cations, organic amines, or mixtures thereof, at a pH range above about 9, for a period of time to provide a distribution of the cations within the ion exchange material. The preferred cations are alkali metal cations, ammonium cations, organic amines, or mixtures thereof. The ion exchange material preferably contains about 0.3 mol percent to about 10 mol percent of alkali metal cations. A plurality of fractions of the solution is collected as the solution passes through the ion exchange material, preferably in a column. This process is particularly preferred for separating rare earth ions and especially lanthanum and neodymium. It is particularly preferred to purify lanthanum to contain less than 0.1 ppm, preferably less than 0.01 ppm, of neodymium.
The ion exchange material is preferably porous glass. The porous glass is prepared by a process comprising producing a base glass from a melt which contains 40 to 80 mol percent of silica and between 0 and 35 mol percent of one or more transition metal oxides selected from the group consisting of the transition metals of Groups IVa, Va, VIa, VIIa and VIII of the Periodic Table and of the actinides, separating the base glass by heat treatment into a least a soluble phase and an insoluble phase, and leaching out the soluble phase to obtain a porous glass containing at least 50 mol percent silica and preferably at least 0.2 mol percent of the transition metal oxides. Alternatively, the porous glass is prepared by melting a base glass which contains 40 to 80 mol percent silica, separating the base glass by heat treatment into at least a soluble a soluble phase and an insoluble phase, leaching the soluble phase and treating the phase separated glass with a solution of one or more salts of said transition metals of Groups IIIa, IVa, Va, VIa, VIIa, VIII, Ib and IIb of the Periodic Table, aluminum, gallium, indium, thallium, tin lead, bismuth, beryllium and the actinides.